Any therapeutic measure which results in the reduction in size of the ischemic border-zone which surrounds a myocardial infarction would represent a major contribution in the clinical management of patients with myocardial infarction. The idea that by pharmacologic or mechanical intervention the extent of the infarction may be reduced is very attractive. A study was undertaken (1) to follow in vivo the progression of myocardial infarction for four days after ligation of the first major lateral branch of the left circumflex coronary artery by continuous mapping of the infarcted area with epicardial electrocardiograms and (2) to determine the effects of the administration of Propranolol and intraaortic balloon pumping on the size of the infarct. The results of the study show that the administration of Propranolol significantly reduces the size of the infarct while intraaortic balloon pumping does not in the experimental model used in this study.